We have attempted to preserve Islets of Langerhans at cryogenic temperatures. The Islets were obtained from adult (150-250 grams) Long Evans rats, using a collagenase digestion procedure followed by isolation using either hand picking or Ficoll gradients. Isolated Islets were organ cultured for two days and then tested for their ability to secrete insulin in response to a graded challenge of glucose, as measured by radioimmunoassay for insulin production. Islets cooled to and seeded at minus 5 degrees C in the presence of 1M DMSO retained their morphological integrity and their ability to respond normally to a glucose challenge. After freezing in the presence of 1.0 M DMSO at approximately minus 5 degrees C per minute to minus 196 degrees C, or using a simplified two step cooling procedure and warming at approximately 10 degrees C per minute, more than 75% of the Islets retained their morphological integrity in organ culture for two weeks or more. Measurements of insulin production in isolated Islets indicate that a high percentage of the prefreezing level of glucose sensitive insulin release can be maintained after freezing. The optimal freezing parameters are currently being investigated. If high survival can be obtained, then the potential exists for overcoming HLA incompatibility problems in humans, thus paving the way for transplantation of isolated Islets into diabetic patients.